1. Field of the Invention
The present invention generally relates to printing, and more particularly, to a method and apparatus for reorienting a printable material, such as printed signatures, in a rotary offset printing press.
2. State of the Art
Known printing presses, such as rotary offset printing presses, often include some mechanism for reorienting a stream of signatures which have been severed from a web of printable material. A conventional reorienting mechanism is configured using one or more walls for initiating what is known in the art as a "bump turn".
To effect a conventional bump turn, a signature is initially transported with a first velocity vector in a first direction. The signature is transported into a wall which ideally decelerates the original velocity vector of the signature to zero, such that a new transport belt can accelerate the signature with a new velocity vector (that is, a new direction) instantaneously. However, in actuality, the reduction of the original velocity vector to zero and the simultaneous establishment of a new velocity vector in the new direction does not occur instantaneously for a number of reasons. For example, frictional forces acting on the signatures prevent an instantaneous transition from the original velocity vector to the new velocity vector. As such, conventional bump turns suffer significant disadvantages.
Among the disadvantages of conventional bump turns are their unreliability in initiating a new velocity vector. In addition, bump turns can result in damage to the signatures and jamming of the press. Moreover, where the stream of signatures are to be maintained in a consistent, shingled stream, conventional bump turns are unable to preserve accurate alignment of lateral edges of the stream, and are unable to maintain an accurate and reliable pitch (i.e., spacing) between adjacent signatures in the stream.
Attempts to address the deficiencies of conventional bump turns include using an in-running nip in conjunction with a partial bump turn. For example, a document entitled "Baldwin Stobb PowerTurn 260.TM." describes a signature stream reorientation device available from Baldwin Stobb of San Bernardino, Calif. wherein a conventional wall interface is used in conjunction with a rotating drum oriented at 45.degree. to the incoming signatures. The signatures are wrapped around the drum and then directed into the wall of the partial bump turn. Because a partial bump turn is still included in the device, the potential for damage to the signatures or jamming of the press still exists. In addition, the use of a partial bump turn can affect the quality of the shingled stream of signatures.
Accordingly, it would be desirable to provide a method and apparatus for reorienting printable material in a printing apparatus such that the reliability of the reorientation can be assured. In so doing, it would be desirable to reduce or eliminate the potential for causing damage to the signatures or jamming of the press, while at the same time, maintaining or improving the quality (e.g., pitch and alignment) of the output stream of signatures.